A laser anneal method is generally known as a method of producing a semiconductor thin film for forming a semiconductor layer of a thin film transistor (hereinafter referred to as “TFT”). The laser anneal method comprises the steps of forming an amorphous semiconductor film or a microcrystalline semiconductor film on a substrate made of glass or the like, and irradiating the film with laser beams for crystallization to give a polycrystalline semiconductor film. Usually this method is called a crystallization process.
Argon laser, KrF and XeCl excimer laser are generally used as a light source for laser beams to be employed in the crystallization process. The TFT produced by the foregoing method is generally called a low-temperature poly Si-TFT since Si is mainly used as a semiconductor and the process is performed at a temperature below the melting point of glass used as the substrate.
Conventional TFT liquid crystal display devices generally include a TFT having a semiconductor layer formed of amorphous silicon, and is provided with a circuit member for driving the pixels which is of the type having IC chips fixed to the periphery of an image plane. On the other hand, even a driving circuit can be produced by use of the low-temperature poly Si-TFT using a TFT formed on a glass substrate. That is, a region outside an image plane can be reduced at an outer periphery of a panel of a liquid crystal display device which is generally called a picture frame and a more elaborate dot-pitch liquid crystal display device can be produced. Various kinds of semiconductor circuits can be formed on a glass substrate by use of a low-temperature poly Si-TFT having improved performance. That is, the so-called system-on-panel (SOP) can be realized. Moreover, with use of a low-temperature poly Si-TFT, an EL display device can be produced by switching an EL display element.
However, the low-temperature poly Si-TFt poses the following problems.
(1) The crystals in a polycrystalline silicon thin film thus formed have a small size so that due to low mobility of electron, response capability and the like are deteriorated in producing a TFT.
(2) In a TFT, numerous grain boundaries of silicon crystals may be present in a boundary between a lightly doped drain area (hereinafter referred to as “LDD area”) or an offset area and a channel area or in its vicinity. In this case, a large number of crystalline defects and dangling bonds exist in the vicinity of the grain boundary so that the performance is deteriorated when the TFT Is allowed to execute a switching operation continuously for a long time or repeatedly many times, resulting in impairment of reliability.
(3) In producing a TFT or a display device, no means is available for determining the positional relationship between the crystals of silicon thin film and TFT pattern, so that it is impossible to determine the position of grain boundary of silicon crystal with respect to the TFT. This results in irregularities of performance in producing a TFT.